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Bureau of Mines Information Circular/1987 



True Flammable Gas Detecting System 



By J. E. Chilton and T. Kubala 




UNITED STATES DEPARTMENT OF THE INTERIOR 





t EaAJ**** *f ' / M^* ) 



Information Circular 9163 



True Flammable Gas Detecting System 



By J. E. Chilton and T. Kubala 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 

David S. Brown, Acting Director 



YlOe 



°fi 




Library of Congress Cataloging in Publication Data: 



^ a tf 



{\t> 



H\ 



Chilton, J. E. 

True flammable gas detecting system. 

(Information circular / United States Department of the Interior, Bureau of Mines ; 9163) 

Supt. of Docs, no.: I 28:27: 9163. 

1. Gas-detectors. 2. Mine gases. I. Kubala, T. E. (Theodore E.). II. Title. III. Series: In- 
formation circular (United States. Bureau of Mines) ; 9163. 



TN295.U4 



[TN305] 



622 s 



[622'.8] 



87-600229 



CONTENTS / 

Page 

Abstract 1 

Introduction 2 

Acknowledgment 3 

Combustible gas measurement techniques 3 

Designing the true flammable gas detecting system 7 

Recommended maintenance and operating procedures 9 

Summary and conclusions 10 

Appendix 11 

ILLUSTRATIONS 

1. Explosive mixtures of CH4 and O2 in air 2 

2. Rik.en-18 light refractance methanometer response to gases 4 

3. Sound velocity measurements in gas/air mixtures 4 

4. Thermal conductivity measurements in gas/air mixtures 4 

5. Catalytic heat-of-combustion sensor response to CH 4 5 

6. MX240 methanometer response to CH 4 -air mixture 5 

7. MX240 response to 2.5 vol pet CH 4 with low O2 concentration 5 

8. Methanometer response to H 2 mixtures 6 

9. Model 502 response to CH 4 , H 2 , and CO 6 

10. MX240 response to CH 4 , H 2 , and CO 6 

11. Proposed gas dilution technique 7 

12. Prototype gas diluter using two peristaltic pumps 7 

13. Model 502 response to CH 4 using the gas dilutor 8 

14. Model 502 response to H 2 using the gas dilutor 8 

15. Lightweight true flammable gas detector 9 

TABLE 

1. Gas composition of sealed coal mines 3 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN 


THIS REPORT 


in 


inch 


mL/rev 




milliliter per revolution 


L 


liter 


m/s 




meter per second 


L/min 


liter per 
minute 


pet 
psig 




percent 

pound per square 


lb 


pound 






inch, gauge 


mL 


milliliter 


vol pet 




volume percent 



TRUE FLAMMABLE GAS DETECTING SYSTEM 

By J. E. Chilton'and T. Kubala 2 



ABSTRACT 

The Bureau of Mines has developed a portable flammable gas detecting 
system for estimating the flammable gas concentrations in a mine con- 
taining one or more combustible gases at concentrations up to 100 vol 
pet. With this portable detecting system, mine rescue or recovery per- 
sonnel can quickly determine if they are working in a potentially explo- 
sive gas atmosphere and either leave the area or make appropriate 
changes in the fresh air ventilation of the mine to render the area 
safe. The detecting system uses a commercial, intrinsically safe, flam- 
mable gas detector with a range from to 5 vol pet methane (CH 4 ). The 
flammable gas detector has a catalytic heat-of-combustion sensor that 
can detect all flammable gases that would be encountered in a mine in- 
cluding methane (CH 4 ), ethane (C 2 H 6 ), carbon monoxide (CO), and hydrogen 
(H 2 ). To determine the mine gas concentration, the rescue worker di- 
lutes a sample with fresh air carried within the detecting system. The 
concentration of the diluted sample is then measured with the permis- 
sable flammable gas detector. The actual mine gas concentration is cal- 
culated from the product of the indicated diluted gas concentration and 
the system dilution ratio using a pocket calculator or the nomograph on 
the side of the unit. 



^Research chemist. 

o . 

■^Research physicist. 
Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



In a mine emergency caused by fire, ex- 
plosion, methane inundations, or massive 
roof or ground disturbance, flammable 
gases are released from coal or rock 
strata or are produced from coal and wood 
fires or CH 4 explosions. In a rescue 
operation, the rescue teams must deter- 
mine if a mine section that the teams are 
entering contains flammable gases at a 
concentration which is potentially explo- 
sive. Rescue teams now carry portable 
gas instruments (methanometers and oxygen 
meters) that indicate combustible gas 
concentrations from to 5 vol pet CH 4 
and/or oxygen (0 2 ) concentrations from 
to 25 vol pet in the mine atmosphere. 
The methanometers, typically with cata- 
lytic heat-of -combustion sensors, are not 
adequate for detecting flammable gas con- 
centrations over the entire explosive 
range (from 5 to 15 vol pet CH 4 , for 
example), or for measuring flammable gas 
concentrations in gas mixtures with low 
2 concentrations. When operating in 
high concentrations of flammable gases or 
in low 2 concentrations, the rescue 
teams also take bottle samples of the 
mine air for subsequent analysis at a 
surface-sited gas chromatograph. Unfor- 
tunately, this gas composition informa- 
tion is not immediately available to the 
rescue teams while they are working 
within the potentially hazardous mine 
site. 

The explosive or flammable range for 
mixtures of CH 4 with fresh air (20.9 vol 
pet 2 ) at room temperature and pressure 
is from 5 to 15 vol pet CH 4 . CH 4 con- 
centrations above the upper explosive 
limit can form flammable mixtures when 
diluted with fresh air. The initial 2 
concentration in the CH 4 mixture is also 
a factor in determining which gas mix- 
tures are flammable as indicated in fig- 
ure 1. Nitrogen (N 2 ) mixtures within the 
left-hand area of the graph are nonflam- 
mable and cannot form an explosive mix- 
ture if diluted with fresh air. CH 4 
mixtures within the right-hand area of 
the graph can form explosive mixtures 
when diluted with fresh air. Special 
precautions must be taken to ensure that 
there are no sources of ignition in the 



mine within the area being ventilated if 
CH 4 concentrations should be in the ex- 
plosive area or in the right-hand area of 
the graph (fig. 1). Mine atmospheres with 
CH 4 concentrations approaching 100 vol 
pet have been encountered during rescue 
or recovery efforts in coal mines that 
had been sealed for several months. Ac- 
curate measurement of CH 4 concentrations 
in the range from 5 to 15 vol pet and 
greater are necessary to avoid potential 
mine explosion hazards when ventilating 
the mine during the rescue and recovery 
operations. 

In coal mine fires and explosions, com- 
bustible gases in addition to CH 4 may be 
formed. The combustible gases found in 
sealed mines include H 2> CO, and C 2 H 6 
(table 1). The mine rescue teams clearly 
need gas instrumentation that can ac- 
curately measure the concentrations of 
combustible gases found in mines. The 
instrument should be able to measure 




4 8 12 16 20 

METHANE, vol pet 

FIGURE 1.— Explosive mixtures of CH 4 and 2 in air. 



CH 4 above 5 vol pet concentration; to 
measure CO, H 2 , and other hydrocarbons as 
well; and the measurements should be 



accurate in gas 
concentration. 



mixtures with any 2 



TABLE 1. - Gas composition of sealed coal 
mines, volume percent 



Methane (CH 4 ) 

Hydrogen (H 2 ) 

Carbon monoxide (CO) 

Ethane (C 2 H 6 ) 

Carbon dioxide (C0 2 ) 
Oxygen (0 2 ) 



Mine A 



10 days 45 days 



19 
.3 

.4 

.2 

.4 

15.4 



95 
.002 
.001 
1.4 
.5 
.4 



Mine B, 
39 days 



0.5 

1.6 

1.7 



8.9 

4.3 



Mine C, 
2 days 



26 
2.5 
4.2 

.04 
4.0 
4.0 



ACKNOWLEDGMENT 



Recommandations of specific gas de- 
tectors and discussions of system design 
alternatives with George H. Schnakenberg, 



supervisory physical scientist, Pitts- 
burgh Research Center, Pittsburgh, PA, 
were invaluable throughout this work. 



COMBUSTIBLE GAS MEASUREMENT TECHNIQUES 



Combustible gases may be detected by 
physical measurements such as light re- 
fraction, sound velocity, and thermal 
conductivity; and by chemical measure- 
ments such as the heat of combustion pro- 
duced by the reaction of flammable gases 
with 2 . Each of these methods has de- 
monstrated a response to specific combus- 
tible gases; however, the physical 
methods may also respond to noncombus- 
tible or interferent gases such as C0 2 , 
which are often present in mine gas mix- 
tures. Interference to the responses of 
the combustible gases may be additive, 
which may lead to an indication of a con- 
centration greater than actually present, 
or subtractive, leading to an indication 
of a combustible gas concentration lower 
than actually present. Both of these 
conditions are undesirable, in particu- 
lar, gas measurements with negative 
interferences may cause unsafe mine work 
operations in potentially explosive gas 
mixtures. 

Infrared (IR) measurement techniques 
have not been considered for this ap- 
plication because not all combustible 
gases, for example H 2 , can be detected by 
an IR technique; furthermore, many IR 
instruments are not portable, and IR 



technology is two to six times more 
expensive than the other techniques under 
consideration. 

The typical response of a gas detector 
(Riken model 18^) based on measurement of 
the index of refraction of light to CH4, 
C 2 H 6 , and H 2 gas mixtures in air is shown 
in figure 2. The instrument response to 
H 2 is negative. Mixtures of combustible 
gases including H 2 would cause negative 
error in the indication owing to the sum- 
ming of the negative and positive re- 
sponses of the individual gases. The re- 
sponse to C0 2 (not shown) is equal to 
that of CH4, therefore in mixtures with 
significant amounts of C0 2 the indicated 
concentration will be incorrectly high. 

The velocity of sound through mixtures 
of two gases, CH 4 and C0 2 , with air is 
given in figure 3. H 2 response, not 
shown, is six times greater than the CH 4 
response. Gas mixtures with both H 2 and 
CH 4 would thus cause an incorrectly high 
response. On the other hand, since the 
response to C0 2 is roughly the negative 
of that of CH 4 , mixtures of gas with 



•^Reference to specific 
not imply endorsement by 
Mines. 



products does 
the Bureau of 



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Hydrogen 



12 3 

GAS CONCENTRATION 
IN AIR, vol pet 

FIGURE 2.— Riken-18 light refractance methanometer 
response to gases. 

10 vol pet of both CH 4 and C0 2 would 
indicate a zero amount of CH4. 

The gas response of a thermal conduc- 
tivity detector with respect to air is 
indicated in figure 4. As is the case 
for the velocity of sound measurements, 
the response to H 2 is much greater than 
that of CH4. For example, the MSA model 
510B, 4 portable methane detector, has a 
response to H 2 , 5.4 times the response to 
CH 4 so the measurement of mixtures of H 2 



440 



420 



400 



t 380 

o 

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o 

CO 



340 



320 



300 




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_L 



20 40 60 80 

GAS IN AIR, vol pet 



100 



FIGURE 3.— Sound velocity measurements in gas air- 
mixture. 




20 40 60 80 

GAS IN AIR, vol pet 



See footnote 3. 



FIGURE 4.— Thermal conductivity measurements in gas air- 
mixtures. 



and CH4 will result in positive error. 
In addition, CO2 may cause negative er- 
rors in the CH4 determinations. The de- 
tector response to CO is close to that 
for air, and flammable gas mixtures 
with percentage quantities of CO would 
not be detected. 

The commercial flammable or combustible 
gas detectors used in mines have cata- 
lytic heat-of-combustion sensors. The 
gas that is being measured is reacted 
with O2 from air on the catalytic surface 
of the sensor and the heat generated by 
this oxidation causes both the tempera- 
ture and the resistance of the sensor to 
rise. This increase in resistance is 
converted to a voltage change propor- 
tional to the gas concentration for dis- 
play on an analog or digital meter by the 
electronic circuit within the detector. 

The response of a typical catalytic 
heat-of-combustion sensor to CH4 is given 
in figure 5 as a function of the CH4 con- 
centration. The response is linear below 
the maximum response value 10 vol pet CH4 
where stoichiometric quantities (2 mole- 
cules of O2 for each molecule of CH4 ) of 
CH4 and O2 react. The response falls for 
CH4 concentrations greater than 10 vol 
pet because insufficient O2 is available 
for complete oxidation of the CH4 . The 




20 40 60 80 

METHANE CONCENTRATION, vol pet 



100 



normal range for commercial CH4 detectors 
using the catalytic heat-of-combustion 
sensor is from to 5 vol pet CH4. The 
linear response in this range for a 
typical methanometer (MX240) is shown in 
figure 6. 

The effect of low O2 concentrations on 
the methanometer response is given in 
figure 7 for measurements conducted in 




12 3 4 5 

METHANE CONCENTRATION, vol pet 

FIGURE 6,— MX240 methanometer response to CH 4 -air mix- 
ture. 



3.0 



2.5- 



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L 




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5 10 15 20 25 

OXYGEN CONCENTRATION, vol pet 



30 



FIGURE 5.— Catalytic heat of combustion sensor response 
to CH 4 . 



FIGURE 7.— MX240 response to 2.5 vol pet CH 4 with low 2 
concentration. 



2. 5 vol pet CH 4 mixtures. The response of 
this methanometer to the 2. 5 vol pet CH 4 
concentration was within the tolerence 
required by 30 CFR 22. 7d(2) in O2 concen- 
trations from 20.9 vol pet (fresh air) to 
7 vol pet. The lower limit for the O2 
concentration of a given CH 4 mixture for 
an accurate CH 4 response may be expected 
at an O2 concentration equal to twice the 
CH 4 concentration, that is 5 vol pet O2 
for 2.5 vol pet CH 4 . The 2:1 ratio of 2 
to CH 4 is required for complete reaction 
to form C0 2 and water (H 2 0). The low re- 
sponse to CH 4 below 7 vol pet O2 may be 
from significant CH 4 reaction to form CO 
with a low heat of reaction. In gas mix- 
tures with the O2 concentration approach- 
ing zero, the catalytic sensor will not 
respond to CH 4 or any other flammable 
gas. 

The methanometers with catalytic heat- 
of-combustion sensors will respond to all 
combustible gases. For example, in fig- 
ure 8 the response to H2 mixtures is 
shown for the MX240 and M502 methano- 
meters. The responses indicated in vol- 
ume percent CH 4 were measured with the 
H 2 concentrations expressed in volume 



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12 3 4 

HYDROGEN CONCENTRATION, vol pet 

FIGURE 8.— Methanometer response to H 2 mixtures. 



percent. If the flammable gas concentra- 
tion is expressed as percent lower ex- 
plosive limit (LEL) and the measuring 
instrument response is given in percent 
LEL, the determination of total combus- 
tibility of a gas mixture can be made 
with these methanometers. Figures 9 and 
10 show the results of measurements with 
CO, CH 4 , and H2 combustible mixtures for 
both the MX240 and M502 methanometers. 
They are read as percent LEL by multiply- 
ing the reading in volume percent CH 4 by 
20; e.g. , 1 vol pet CH 4 in air is a 20- 
pct LEL mixture. The responses of the 



CO 

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CO 



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/ 


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1 1 1 1 1 



20 40 60 80 100 120 140 

GAS CONCENTRATION, pet LEL 

FIGURE 9.— Model 502 response to CH 4 , H 2 , and CO. 




20 40 60 80 

GAS CONCENTRATION, pet LEL 

FIGURE 10.— MX240 response to CH 4 , H 2 , and CO. 



00 



M502 methanometer to H 2 is less than the 
responses to CH 4 or CO. The low H 2 re- 
sponse for the M502 may be caused by par- 
tial reaction of H 2 on the reference or 



inactive element in the sensor. The re- 
sponses of the MX240 are approximately 
equal for CH4, H2, or CO when all concen- 
trations are expressed in percent LEL. 



DESIGNING THE TRUE FLAMMABLE GAS DETECTING SYSTEM 



The proposed true flammable gas detect- 
ing system should have certain character- 
istics in order to be used in postdiaster 
operations in gassy mines. The methano- 
meter must be intrinsically safe and ap- 
proved for use in CH 4 mixtures with air 
per Mine Safety and Health Administration 
(MSHA) specifications. Any future need 
for certification of electrical equipment 
for operation in CH4-H 2 -air mixtures 
should be addressed by MSHA. The meth- 
anometer should have a catalytic heat- 
of-combustion sensor so that the meter 
will respond to all combustible gases 
found in mines; for example, CH 4 , C 2 H 6 , 
H 2 and CO. In order for the methanometer 
to work properly in mine atmospheres with 
minimum 2 , the mine sample must be di- 
luted with fresh air to supply enough 2 
for total combustion and to reduce the 
combustible gas concentration to a value 
to be measured by the methanometer with a 
0- to 5-vol pet CH 4 range. The system 
should be small size and light weight for 
carrying over the shoulder while travel- 
ing in the mine. 



Syringe 



10 mL 



3-way 
stopcocks 




Methanometer 



One gas dilution system considered for 
use with this concept is shown in figure 
11. This system used a 10-mL syringe for 
taking a gas sample and a 50-mL syringe 
for diluting and mixing the gas sample 
with fresh air. The sample was then 
transferred to the methanometer for mea- 
surement of the combustible gas content. 
This system required the manipulation of 
two three-way stopcocks for operation. 
The proper sequencing of the valves would 
be difficult to accomplish during mine 
rescue operations; for this reason, this 
dilution system was rejected. 

The dilution system finally selected is 
shown in figure 12. This system uses two 
peristaltic hand-driven pumps for the 
dilution and transfer of the gas mixture. 
The mine gas is drawn into one pump con- 
taining tubing with a small sample volume 
and the fresh air sample is drawn into 
the other pump containing tubing with a 
large sample volume. The two pumps are 
connected by a common shaft which is 
hand-cranked. By this coupling of the 
pumps, a given volume of mine gas and 
fresh air is pumped for each turn of the 
handle and the dilution volume is con- 
stant, dependent only on the diameters of 
each of the tubes. 



Valve 



Mine air 



FIGURE 11.— Proposed gas dilution technique. 




FIGURE 12.— Prototype gas diluter using two peristaltic 
pumps. 



The gases are mixed and transferred to 
the methanometer for measurement of the 
diluted combustible gas concentrations. 
In operation, a sample of gas is fed into 
the sensor cavity, the sensor is then 
electrically powered, and the gas sample 
is completely reacted. The M502 analog 
meter response rises to a maximum value, 
then falls as the combustible gas in the 
cavity is consumed. The methanometer 
selected for this system was the model 
502 because it requires a gas sample of 
only about 10 mL volume for a gas mea- 
surement. The M502 methanometer is cali- 
brated so that the maximum value noted on 
the meter is equal to the measured gas 
concentration. 

Tests were also run using the MX240 
methanometer and over 300 cranks of the 
pump handle were required to give a con- 
stant response. This unit is continually 
powered and requires a large gas sample 
volume for an indication. The MX240 unit 
cannot be used with this dilution system 
since the system contains only a limited 
volume of fresh air for making the gas 
dilutions. 

The first prototype system was assem- 
bled in an aluminum box containing a 
fresh air bag, the methanometer, the two 
peristaltic pumps, a rubber squeeze bulb 
for filling the fresh air bag, and inter- 
connecting tubing. This box was approxi- 
mately 6 by 8 by 9 in and weighed 7.5 lb. 
The dilution system was tested using CH 4 - 
air mixtures (fig. 13). About seven 
cranks of the handle driving the pumps 



were needed to deliver sufficient gas for 
measurement. The average gas dilution 
ratio was 28.3 for the tests at different 
CH 4 concentrations. Gas concentrations 
from 100 to 10 vol pet CH 4 can be mea- 
sured with this system. System response 
to various H 2 -air mixtures using the 
model 502 detector are shown in figure 
14. For these measurements, the poly- 
vinylchloride (PVC) tubing in both pumps 
was replaced because of wear, and a new 
dilution ratio of 26. 7 was obtained for 
100 vol pet H 2 , and then the same ratio 
was measured for the dilution of CH 4 . 

The first prototype unit was demon- 
strated to the MSHA mine rescue teams. 
The teams recommended that the unit be 
made smaller and lighter in weight in 
view of the considerable weight and vol- 
ume of other equipment that must also be 
carried in mine rescue missions. They 
also recommended that the dilution ratio 
be reduced to more accurately measure the 
explosive region of CH 4 , 5 to 15 vol pet 
in air. A second gas detector unit has 
been constructed which is 8 by 6 by 5 in 
and weighs 5 lb. The following design 
changes were implemented: 

• The fixed length crank handle has 
been replaced with a folding handle. 

• Two of the pumps were changed to a 
different sized tubing to obtain a 14:1 
dilution ratio. 

• Silicone rubber tubing was used in 
place of the PVC plastic tubing in the 
pumps to improve tubing life and thus 
obtain a more constant dilution ratio. 




5 10 15 

PUMP CRANK, turns 



20 



Pump 

dilution 

ratio 

28.6 



28.5 

27.7 
27.6 

28.7 
28.5 



FIGURE 13.— Model 502 response to CH 4 using the gas 
dilutor. 



_ 4 



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in 

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CO 
Ul 

cc 

Ld 

< 

X 



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IT) 



- 


I 

- — • 


1 

• 


1 

1 


— 


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// 


■ 


=a 


— 


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f / 




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1 


1 


1 



5 10 15 

PUMP CRANK, turns 



H2, 
vol pet 


Pump 

dilution 

ratio 


100 


26.7 


75 


26.3 


50 


27.0 



25 



20 



27.8 



FIGURE 14.— Model 502 response to H 2 using the gas 
dilutor. 



Four of the lightweight prototype units 
have been designed and fabricated. Fig- 
ure 15 shows the lightweight flammable 



gas detector. The air pumps and the 
fresh air bag are placed in the space 
under the permissible methanometer. 



RECOMMENDED MANTENANCE AND OPERATING PROCEDURES 



While in storage, the flammable gas de- 
tecting system should be placed at a 
central location with the manufacturer's 
charger connected to the permissible 
methanometer when required to keep the 
batteries charged. In operation, the 
flammable gas detecting system and a 
calibration kit would be carried to the 
mine site. The calibration kit contains 
a CH 4 -air mixture to calibrate the 
methanometer and a CH 4 or CH4-N2 mixture 
to measure the dilution ratio. The air 
bag should be filled by operating the 
hand squeeze bulb while the unit is in 
fresh air. The air bag position under 




f h. 



FIGURE 15.— Lightweight true flammable gas detector. 



the methanometer air is visible through 
the acrylic cover of the unit. 

Calibrate the methanometer with the 2. 5 
vol pet CH 4 standard gas mixture, and 
measure the gas dilution ratio using a 
100-vol pet or a 50-vol pet CH4 mixture 
depending on the unit's recorded dilution 
ratio. Record the methanometer calibra- 
tion data in a permanent log book along 
with the record of the dilution ratio and 
also post it on the dilution system 
case. 

A nomograph for direct conversion of 
the indicated gas concentration for the 
diluted sample to the calculated value 
for the original mine gas concentration 
is placed on the side of each unit. The 
nomograph was initially constructed using 
the originally measured gas dilution 
ratio, but if subsequent measurements are 
sufficiently different, the nomograph 
should be redrawn to fit the new dilution 
ratio value. 

In the mine, operate the unit by turn- 
ing the handle 7 to 10 turns and taking a 
reading. If possible, repeat the mea- 
surement to verify the accuracy of the 
initial reading. The original mine gas 
concentration can be determined from the 
meter indication by use of the nomograph 
on the side of the case. 

The air bag has a sufficient volume for 
over 30 measurements, and may be refilled 
in the mine at a verified fresh air site. 
Verify mine fresh air sites by measuring 
the CH 4 gas concentration directly with 
the methanometer and verifying that the 
CH 4 concentration is zero. In addition, 
use a permissible O2 meter to verify that 
the 2 concentration is 20.9 vol pet. 

Since the PVC plastic or silicone rub- 
ber tubing loses its resiliency with con- 
tinued use, replace the pump tubing after 
prolonged use or when a significant 
change is found in the dilution ratio. 
New tubing may have a slightly different 
inner diameter even if it comes from the 
same tubing lot. Measure the dilution 
ratio before each use by testing the 



10 



dilution system with a suitable calibra- 
tion gas. For pump combinations with the 
dilution ratio of 20 or greater, a cali- 
bration gas of 100 vol pet CH 4 can be 
used. For example, if 100 vol pet CH 4 is 
diluted and a methanometer reading of 3. 5 
vol pet CH 4 is obtained, the dilution 
ratio is 100/3.5 or 28.6. 

This system was named a true flammable 
gas detecting system because the cata- 
lytic heat-of-combustion sensor will re- 
spond to all flammable gases in the 
diluted sample. The accuracy of the re- 
sponse from the M502 methanometer is high 
for CH 4 or CO, but the response for H 2 is 
only 60 vol pet of the true H2 concen- 
tration. Thus, mixtures of flammable 
gases with H2 will have a response less 
than the true response. For example, a 



mixture of 5 vol pet CH 4 , 4 vol pet CO, 
and 4 vol pet H 2 run in a true flammable 
gas detection system with a dilution 
ratio of 28.6 would read 0.34 vol pet CH 4 
on the M502 meter. 

The value of 0. 34 vol pet CH 4 for the 
diluted gas corresponds to an undiluted 
gas sample of 9.6 vol pet CH 4 or 192 pet 
LEL for the sum of the flammable gas 
LEL's (5 vol pet CH 4 = 100 pet LEL). 
This calculated value of 192 pet LEL for 
the M502 measured sum of the gas constit- 
uent LEL is 17 pet less than the true sum 
of the LEL's for this mixture, which, is 
232 pet LEL. 

A list of parts for construction of the 
true flammable gas detection system is 
given in the appendix. 



SUMMARY AND CONCLUSIONS 



A true flammable gas detecting system 
was developed that uses a catalytic heat- 
of-combustion combustible gas detector to 
measure the combustible gas concentration 
of a mine gas sample diluted by fresh 
air. The dilution equipment consists of 
two peristaltic hand-operated pumps that 
supply a fixed ratio of fresh air to sam- 
ple gas. The dilution ratio is set by 
the volumes of the flexible tubing within 
the pumps. Two units were prepared with 
approximate dilution ratios of 14:1 for 
accurate measurement of CH 4 within the 
5- to 15-vol pet explosive range. The 
dilution ratio for these units is deter- 
mined by use of a standard 50 vol pet 
CH 4 -N2 gas mixture. In addition, two 
units were fabricated with approximate 
dilution ratios of 28:1, which can be 
used to measure gas mixtures containing 
up to 100 vol pet CH 4 concentrations. 
The dilution ratios for these units is 
determined by calibration with a standard 
gas of 100 vol pet CH 4 . For all units, 
the dilution ratios must be periodically 
measured and, if found to vary from ini- 
tial values, new flexible tubing should 
be placed in the air pumps. The fresh 
air is contained within a flexible plas- 
tic bag and the bag is filled before en- 
tering the mine. The 1-L bag contains 



enough fresh air for over 30 gas con- 
centration determinations. The catalytic 
heat-of-combustion sensor was chosen for 
measuring the flammable gases because it 
responds to all combustible gases includ- 
ing H 2 , CO, CH 4 , and C 2 H 6 , that could be 
encountered in mines or in mine fires or 
explosions. Other sensors using thermal 
conductivity, infrared absorption, and 
velocity of sound systems are available 
to measure CH 4 in concentrations up to 
100 vol pet. These sensors could be used 
in mine gas detectors if only CH 4 gas in 
air were to be measured. In coal mine 
fires or explosions, CH 4 is found togeth- 
er with other combustible gases such as 
H 2> C 2 H6, and CO, and the catalytic heat- 
of-combustion sensor is of more general 
application. This flammable gas detect- 
ing system may be especially useful for 
quickly providing combustible gas infor- 
mation in rescue or recovery operations. 
The use of theses units in mine rescue 
operations by mine workers or mine rescue 
personnel from MSHA or the Deep Mine 
Safety Division of the Pennsylvania 
Department of Environmental Resources, 
Uniontown, PA, is expected to contribute 
to the safety of the mine rescue work and 
to aid the mine recovery efforts. 



11 



APPENDIX 



A parts list for the critical compo- 
nents of the true flammable gas detecting 
system is given. 

• Instrument case . — rectangular box, 
aluminum, 5-in width, 8-in length, and 
6-in height, Zero Corp., 288 Main St., 
Monson, MA 01057. 

• Hand-operated peristaltic pumps . — 
Masterflex pump head R 7013-20 (0.006 
mL/rev) and Masterflex pump head 
R 70015-00 (long shaft) (1.67 mL/rev). 
Cole-Palmer Instrument Co. , 7425 North 
Oak Park Ave., Chicago, IL 60648. 

• Silicone flexible tubing, selected 
dimensions. — Cole-Palmer Instrument Co. 



• Permissible methanometer . — Model 
M502 (Auer) range from to 2 vol pet and 
from 2 to 5 vol pet CH 4 , part No. 460139, 
Mine Safety Appliances , 600 Penn Center 
Blvd., Pittsburgh, PA 15206. 

• Calibration equipment . — Model SGP 
plastic case, disposable steel cylinder 
PN559--2.5 vol pet CH 4 in air (103 L at 
1,000 psig), flow regulator— PN7 15 (0.31 
L/min flow), Alphagaz, Woods Road, P.O. 
Box 149, Cambridge, MD 21613. 

• Specialty gas . — 100 vol pet CH4 or 
50 vol pet CH 4 in N, 14 L at 240 psig, 
regulator for 1-to 25-psig delivery pres- 
sure, Scott Specialty Gases, Route 611, 
Plumsteadville, PA 18949. 



US GOVERNMENT PRINTING OFFICE: 1987 - 605017/60117 



INT.-BU.0F MINES,PGH.,PA. 28583 



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P.O. Box 18070 
Pittsburgh. Pa. 15236 



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